The present invention relates to the inspection of a predetermined area of the surface inner wall, and welds, of a pressurized water nuclear reactor pressure vessel, whereby a method is provided to make such inspection without removal of the lower internals.
A typical pressurized water nuclear reactor includes a generally cylindrical pressure vessel which houses the heat generating reactor core, and a plurality of flow loops through which reactor coolant fluid is circulated. In each loop, coolant that is heated in the core is generally placed in heat exchange relation with a vaporizable fluid which is then used to drive tubine-generators. The coolant is then returned to the pressure vessel. In the pressure vessel there are contained the reactor vessel internals, the functions of which include support of the core components, such as fuel assemblies, and support of core monitoring apparatus. All of the supported load is transmitted through the wall of a core barrel, which surrounds the reactor core, to the reactor vessel. A baffle plate-former assembly is typically provided between the core barrel and the core, which is also supported by the core barrel. Coolant flow generally enters the pressure vessel, passes downwardly through an annulus formed between the core barrel and the pressure vessel wall, and then is turned 180.degree. to flow upwardly through the core and then out of the pressure vessel.
The pressure vessel is normally of a generally cylindrical shape and has a cylindrical pressure vessel wall, which wall has inlet and outlet nozzles for conducting coolant, a hemispherical shaped bottom wall and a removable hemispherical shaped head. The cylindrical pressure vessel wall, because of its size, is normally formed of a plurality of sections that are welded together, and the cylindrical pressure vessel wall is also welded to the bottom head and wall so as to provide an integral primary pressure boundary for the coolant passed through the pressure vessel. The reactor vessel normally is formed as a thick walled carbon steel vessel that has a stainless steel cladding deposited thereon, on the interior surface.
Because of the pressurized state of the coolant within the pressure vessel, the vessel wall and welds therein are examined prior to placing a reactor in service. After the reactor has been placed in service, subsequent periodic examination of the pressure vessel wall and any welds are also required. This is referred to as "inservice" inspection. Normally, a complete examination of the reactor vessel wall and welds is required after an initial ten year period of service. In the event that a minor flaw is discovered in a weld during a routine examination, which flaw or "indication", is of such a minor nature that use of the vessel may be continued, it is necessary to make periodic subsequent inspections of that area at more frequent time intervals, such as each successive forty month period, in order to monitor and detect any aggravation of the flaw initially detected.
Inservice inspection of the inner surface of the pressure vessel wall and welds can be achieved by the removal of the reactor internals, both upper and lower internals, which completely exposes the inner wall surface. In some instances, however, it is disadvantageous to remove the complete reactor vessel internals for inspection purposes. It would be advantageous therefore, if inspection of the inner surface and welds of the pressure vessel could be effected without a need to remove the lower internals package of the reactor.
It is an object of the present invention to provide a method of inservice inspection of the inner surface and welds of a pressurized water reactor pressure vessel without removal of the lower internals of the reactor from the pressure vessel.